1. Field of the Invention
The present invention relates to an optical waveguide and a method of manufacture thereof and more particularly to an optical waveguide for novel functional optical integrated circuits using an optical functional material KTaxNb1−xO3 as the optical waveguide and a method of manufacture thereof and also to a method of manufacturing a crystal film for use with optical communication devices.
2. Description of the Related Art
Intensive research and development efforts are being made from a cost performance point of view to develop optical integrated circuits that integrate on a single substrate optical devices that perform emission, detection, modulation, and multiplexing and demultiplexing of light. This integration technology is expected to reduce electric power, enhance performance and reduce cost of these optical devices.
Conventional optical integrated circuits currently in wide use have a fabrication in which a waveguide structure is formed on a semiconductor substrate using SiO2 and polymers to process an optical signal launched from outside. The waveguide structure refers to a structure comprising an undercladding layer, a waveguide layer formed on the undercladding layer and having a refractive index higher than that of the undercladding layer, and an overcladding layer covering the waveguide layer and having a refractive index smaller than that of the waveguide layer. To realize a function of optical signal processing, the conventional optical ICs change an optical properties of the waveguide material as represented by ordinary and extraordinary refractive indices by applying external fields, such as heat, electric fields, magnetic fields and sound, thereby achieving such functions as multiplexing/demultiplexing optical signals and adjusting a transfer time.
However, since the waveguide materials currently available are limited to SiO2, polymers, semiconductors and a small range of nonlinear crystals, the changing of the optical properties as realized by the method described above is greatly restricted by the characteristic of the waveguide material used, thus imposing limitations on the applicable optical signal processing.
Under these circumstances, the use of a novel waveguide material KTaxNb1−xO3 is being considered. The optical functional material KTaxNb1−xO3 exhibits an optical second-order nonlinear effect. An optical nonlinear constant of this material is 1,200–8,000 pm/V, significantly larger than 31 pm/V which is an optical nonlinear constant of LiNbO3 for example.
Further, since this optical nonlinear effect is attributed to the displacement of positions of constitutional elements by the application of an electric field, the presence or absence of the optical nonlinear effect can be controlled by the application of an electric field.
The material KTaxNb1−xO3 undergoes a ferroelectric phase transition at a composition-dependent Curie temperature of between −250° C. and 400° C. At this Curie temperature as a boundary the material's property changes significantly. For example, its dielectric constant greatly changes from approximately 3,000 to about 20,000. It is possible to create a new optical integrated circuit taking advantage of the ferroelectric phase transition. The Curie temperature varies depending on the composition x of KTaxNb1−xO3, and adding Li to KTaxNb1−xO3 to produce KyLi1−yTaxNb1−xO3 makes it possible to adjust the temperature range.
The fabrication process of an optical waveguide requires steps of first forming a waveguide material film and then performing patterning and etching on the film using photolithography or the like.
The currently used waveguide materials, however, are limited to SiO2, polymers, semiconductors and a small range of nonlinear crystals. Hence, the modification of optical properties as realized by the aforementioned application of heat, electric fields, magnetic fields or sound is greatly restricted by the characteristics of the waveguide material used. The conventional optical ICs therefore have a problem that the applicable range of optical signal processing is very narrow.
Further, the method of manufacturing an optical waveguide using the KTaxNb1−xO3 optical functional material described above also requires the fabrication, process, similar to the conventional one, of forming a film of the waveguide material and patterning the waveguide film by photolithography. Therefore, even in using the novel waveguide material KTaxNb1−xO3, the conventional technology has a problem that the waveguide fabrication process is complex.
Another problem is that, although the waveguide fabrication is essential in obtaining a desired performance, a technique to form waveguides in a KTN crystal has not yet been established. This is attributed to the fact that ions that increase the refractive index and still do not degrade the nonlinear characteristic after diffusion has not been found.
The chemical vapor deposition (CVD) method vaporizes a material containing constitutional components and causes a desired reaction in a gas phase or on a substrate. Forming a waveguide material film by using the CVD method requires a volatile compound containing the constitutional components. In KTN or KLTN, as to the compounds of Ta and Nb, halide and alkoxide have high volatility and can be used as the starting material when the CVD method is applied.
As to K and Li compounds, there is not much information available about the materials which provide sufficient vapor pressures. In the case of K in particular, no material has been known which is effective for use with the CVD method. The essential reason for this is that alkali metal elements such as K and Li tend to be ionized easily and cannot easily be kept in a molecular state necessary for vaporization.